What is the main difference between a heart attack and anxiety? Can heart attack symptoms last for months?

I was diagnosed with anxiety about 5 months ago and am currently taking Zoloft for it. I am a first time mom and my son is 8 months old. Following childbirth, I had eclampsia with seizures. Ever since I had these, I am TERRIFIED of having another one...which doctors say is HIGHLY unlikely! Well, since then I have had bloodwork, 2 MRI's, CAT scan, and all kind of heart tests. I still get lightheaded at times...and my heart feels like it is beating fast and I feel like Im going to faint. I cant stand feeling like this! Do you think the lightheadedness could be due to the Zoloft? Also, there is no way that heart attack symptoms could last this long, right?